Having learnt a bit more about the behaviour of lights in TG2 I went back to basics and started again. I've resigned myself to working with point light sources for now unless someone can tell me how to get a more diffuse light (using lights, not objects)
Attached is a modified version of the basic fill light setup. I split the fill lights into 5, 3 angled fill lights without shadows and a pair of lights at the zenith. The zenith lights server 2 purposes. 1) provide additional lighting from above to give more shaping to the terrain in the shadows 2) an additional light to provide an "ambient shadow" and a variation in colour between normal shadows and deep shadows. This light has a neutral colour in the attached clip file but I'll be experimenting with warmer colours to provide additional contrast in the shadows.
I have at least one more light to add to the mix, plus testing with different GI settings, but this is looking much better than my previous tests (once you stick an object in the scene). With only one light casting shadows the render times are pretty reasonable. I'll do some benchmarking later without the tree as it takes a along time to render.
PS I'm ignoring the effects of the lights without shadows on canyons at the moment as I haven't really done any yet, but if anyone wants to add a test render in a canyon with overhangs (without GI)... feel free to do so
Update: added image with GI (tried Ambient Occlusion, strength on surfaces 0.5) and it appears that GI doesn't apply to objects at the moment (as well as fake stones). While it's obvious that these things will be addressed in time, it's a good thing to keep in mind when attempting serious images with this preview version. For now I'll be restricting my tests to lighting without GI... possibly trying a very small addition of GI for subtle tweaks.
Running a couple of GI comparison renders (default enviro settings, detail 2, quality 4), without the tree (cuts out 3 1/2 hours render time). Occlusion is similar to using fill lights and renders *much* quicker than Illumination. Not the best test scene for GI only comparisons, so I'll repeat this later with a more suitable sample.
